Utilization of infra-red detectors for detecting the presence of fire or explosions has been going on for several years. Various type systems are disclosed in U.S. Pat. Nos. 4,220,857, 3,665,440, 3,931,521, 3,825,754, 3,724,474 and 3,859,520. A system employing a dual chanel detecting scheme is disclosed in our co-pending application entitled OPTICAL FIRE or EXPLOSION DETECTION SYSTEM and METHOD. This system detects the presence of fire by processing received signals emitting from fires in the 4.3 micron and 3.8 micron series. The processing includes circuitry to detect the so-called flicker frequency of the fire.
These detectors are typically located in areas such as air craft hangars, gasoline loading racks, petrochemical plants, on-shore and off-shore oil and gas production sites and the like. The detectors more often than not, are located out of doors, where they are exposed to the elements and whatever pollution there may be in the ambient air. Consequently, the optical windows of these detectors often become occluded with foreign material. This foreign material is often highly absorbent in the ultra-violet portion of the spectrum and somewhat absorbent in the infra-red portion of the spectrum and therefore, can very likely mitigate the ability of the detector to detect a fire. Therefore, it is critical that the sensitivity of the detector be able to be checked.
The inventors are aware of single channel infra-red detectors with built in test lights. Also, ultraviolet detectors exist in which the sensitivity's checked by using some kind of reflection of radiation from an internal light source back into the detector.
Inasmuch as the multiple channel infra-red detector measures the light in each of two or more bands of the infra-red spectrum, any means of checking the sensitivity must provide different levels of light in the two or more bands of the infra-red spectrum which would be analogous to what exists when a flame is present. This represents a particularly difficult engineering problem because the only light sources available that can provide light in the infra-red portion of the spectrum are incandescent lamps. However, infra-red detectors, in order to be useful, must be able to ignore black body radiative sources such as incandescent lamps. Consequently, a technique which uses an incandescent lamp would on the surface be fruitless.
It is therefore a primary object of this invention to disclose a built-in test device for checking the sensitivity of a multiple channel, infra-red detection system.
It is a further object of the invention to employ an incandescent lamp, or other light source with a spectral output appropriate to the spectrum being measured by the detector, to provide a simple test device which will give realistic numerical data on the sensitivity of the detector.